DC/DC power converter control strategy for source protection
A controller (11a) of a DC/DC converter (10a) responsive to power output of a fuel cell power plant (13) operates under a control strategy which determines if fuel cell voltage exceeds a limit, and if so, provided neither fuel cell output current nor DC/DC converter output current is excessive, causes an increase in DC/DC converter duty cycle to thereby increase power demanded from the fuel cell stack. This eliminates the need for conventional voltage limiting to protect fuel cells from corrosion. Digital control loops and state machines are illustrated.
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A DC/DC power converter control strategy includes response to a critical parameter, such as output voltage of a DC power source, such as a fuel cell, to alter the converter output in a manner which a) tends to cause the value of the critical parameter to be below a limiting value, b) but may drive another critical parameter of the source or of the converter below a limiting value.
BACKGROUND ARTDC/DC power converters are used in a variety of power systems, such as fuel cell/battery hybrid power propulsion systems currently in use in operational buses. The DC/DC power converters can either be boost type, if the desired output voltage is higher than the available input voltage, or it can be buck type if the desired output voltage is lower than the available input voltage. DC/DC converters may also be bidirectional, allowing power flow in both directions, or they may be unilateral, allowing power flow in only one direction.
The typical prior art DC/DC converter associated with a fuel cell stack is controlled by a system, which is usually digital, that adequately controls the converter input current (which is the output current of the DC power source e.g., fuel cell stack) and the converter output current and voltage. However, prior strategies have not been effective for managing DC power source output voltage constraints. Fuel cells have a monotonic voltage/current performance curve relationship. At low output power, the voltage of each cell can become sufficiently high to cause corrosion of the cathode and anode catalysts and the carbon catalyst supports. This corrosion causes permanent decay in the performance of the fuel cell.
Heretofore, prevention of fuel cell performance decay, as a result of catalyst and support corrosion at high cell voltages, has typically been prevented by means of a voltage limiting device (VLD) such as an auxiliary resistive load, which is caused to effectively be a variable load by switching it in and out of the circuit by means of an electronic switch controlled by a pulse width modulation (PWM) signal.
Referring to
In
The inputs to the controller of the DC/DC converter are provided on a plurality of signal lines 19-21 as illustrated in
Referring to
If the fuel cell output current does not exceed a corresponding limit, a negative result of test 40 reaches a test 43 to determine if the DC/DC converter output current, Iout, exceeds a related limit. If the converter output current exceeds its limit, an affirmative result of test 43 will reach the step 42 to decrement the duty cycle, causing power to diminish. But if both the fuel cell output current and the converter output current are within limits, then negative results of tests 40 and 43 will reach a step 45 to increase the duty cycle. In other words, if the output currents are both in limit, then when the voltage output is less than the commanded voltage output as indicated in test 38, the duty cycle is increased at step 45 which will increase the output voltage of the DC/DC converter and cause an increase in output power.
If the voltage output of the DC/DC converter exceeds the output voltage command, a negative result of test 38 will reach the step 42 to decrease the duty cycle. This causes a decrease in the output voltage, and a decrease in the output power. Thus, the step 45 pushes the power (increases it) whereas the step 42 causes power to be diminished (not to be pushed). After either of the steps 42, 45, other routines are reverted to through a return point 48.
SUMMARYA control strategy for a DC/DC converter receiving power from a DC power source having a critical parameter with a limiting value, accommodates the critical parameter by adjusting converter output, and therefore altering power delivered by the source in a manner to favorably affect the value of the critical parameter, provided limits of other parameters are not exceeded.
In one example, the critical parameter is cell voltage of a fuel cell power plant wherein a voltage indicative of average or representative cell voltage is utilized to determine whether output power should be increased, thereby causing cell voltage to decrease.
In the disclosed embodiment, rather than using a voltage limiting device, typically with a pulse width modulated switching circuit and a VLD controller, the present control strategy utilizes the controller of the DC/DC converter, with fuel cell voltage as another input, to alter the mode of operation in the event that the voltage of the fuel cell stack becomes too high, to push (boost) the power output to thereby reduce the fuel cell voltage, provided other limits have not been exceeded.
The control strategy can be used with other DC sources employing power conversion by DC/DC converters, such as solar cells. In the general case, the present control strategy can be utilized in any situation where an instruction within the control strategy adjusts the output of the DC/DC converter in a manner which in turn alters a critical parameter of the DC source that provides power to the DC/DC converter.
The control strategy hereof may be used with unidirectional or bidirectional DC/DC converters of either the boost or buck type. The control strategy hereof may be used with DC/DC converters which control either the current output or the voltage output which is provided to a load.
Other variations will become more apparent in the light of the following detailed description of exemplary embodiments, as illustrated in the accompanying drawings.
A system employing the control strategy provided herein is within a fuel cell power plant system 15a illustrated in
The output of the controller 11a in
In
If the cell voltage is too high, an affirmative result of test 54 will reach the tests 40 and 43 to determine if either the output current of the fuel cell or the output current of the DC/DC converter exceeds its corresponding limit. If both tests 40 and 43 are negative, then the duty cycle is increased in a step 45 and the programming reverts to other routines through the return point 48. If pushing power, to lower cell voltage, results in either current exceeding its limit, the result of either test 40 or 43 will change to positive in the next pass of the routine of
If the cell voltage is high but either of the currents are above their limits, an affirmative result of either test 40 or 43 will prevent the duty cycle from increasing, and reach the step 42 which will decrease the duty cycle as described hereinbefore.
But whenever the output voltage of the DC/DC converter exceeds its commanded output voltage, an affirmative result of test 38 reaching the test 54 will result in a decrease in the duty cycle 42 so long as the cell voltage does not exceed its corresponding limit in the test 54.
Thus, by providing one additional control factor in the controller 11a, in accordance with the present control strategy, the apparatus 29-34 of
One possible load 16a for the DC/DC converter operating under the present control strategy is as the primary power for a vehicle drive illustrated in
Either in addition to the energy storage system 74, or in place thereof, there may be a power dissipating device (PDD) 75, which is essentially the same as the voltage limiting device referred to in
Concerning the DC/DC converter 10a operating with the present control strategy, if the tests in the control strategy of the DC/DC converter controller 11a in
Another possible load 16b for a DC/DC converter 10a operating under the present control strategy is as backup power for a telephone exchange shown in
To accommodate interruptions in power provided by the utility grid, a large energy storage system 84 is provided. This system can provide the nominal DC voltage to the electronic devices for relatively short periods of time, and be recharged at the nominal DC voltage by the AC/DC converter 79 once power from the utility grid 82 is restored.
For longer terms of power outages, the fuel cell power plant 15a of
The loads 16a and 16b of
The modality herein has been described as it may be implemented by means of digital routines. However, it may be implemented in other ways, such as by a state machine, as illustrated in
Any time there is a power up or any other reset, as indicated by line 53a in
In
Once in the cell voltage regulating state 98, it will remain there unless the fuel cell voltage returns to a value below its limit. Then, it returns to the DC/DC converter output voltage regulating state 87, as indicated by line 54b. Conventional hysteresis may be provided between the conditions 54a and 54b to prevent hunting between states.
Another manner in which the state machine may advance out of the state 87 which regulates the DC/DC converter output voltage, is if the output current of the DC/DC converter exceeds its limit, as indicated by the line 43a. It then would reach a state 100 in which the DC/DC converter output current is regulated, as shown in
If either the DC/DC converter output voltage (Vout) or fuel cell voltage (Vcel) exceeds a corresponding limit, the state may revert to the state 87 where output voltage is regulated, or it may revert to state 98 where fuel cell voltage is regulated, respectively.
The expression of the modality herein in terms of a state machine does not alter the situation: the modality herein eliminates the need for voltage limiting of any sort, such as power dissipation or storage, since it causes the DC/DC converter to push power whenever necessary to retain the fuel cell voltage below its limit, (unless the DC/DC converter has an excessive output current) as described hereinbefore.
Since changes and variations of the disclosed embodiments may be made without departing from the concept's intent, it is not intended to limit the disclosure other than as required by the appended claims.
Claims
1. A method comprising:
- receiving, by a DC/DC converter operatively coupled to an output of a DC power source, a DC power source voltage output signal indicative of a value of an output voltage of the DC power source;
- determining, by the DC/DC converter, whether the output voltage of the DC power source exceeds a first limiting value based on the received DC power source voltage output signal; and
- responsive to determining the output voltage of the DC power source exceeds the first limiting value, comparing, by the DC/DC converter, the value of an output current of the DC power source to a second limiting value; comparing, by the DC/DC converter, the value of an output current of the DC/DC converter to a third limiting value; and responsive to determining that the output current of the DC power source is below the second limiting value and the output current of the DC/DC converter is below the third limiting value, operating the DC/DC converter to increase power output of the DC power source to decrease the value of the output voltage of the DC power source to a value which is at or below the first limiting value.
2. A method according to claim 1 wherein receiving an output of a DC power source comprises receiving an output of a fuel cell power plant.
3. The method of claim 2 wherein the output voltage of the DC power source is a cell voltage in a fuel cell stack of the fuel cell power plant.
4. An apparatus comprising:
- a DC power source having an output voltage with a first limiting value which should not be exceeded; and
- a DC/DC converter operatively coupled to an output of the DC power source, in operation the DC/DC converter: determines whether the first limiting value of the output voltage of the DC power source has been exceeded, and if so, compares the value of an output current of the DC power source to a second limiting value; compares the value of an output current of the DC/DC converter to a third limiting value; and responsive to determining that the output current of the DC power source is below the second limiting value and the output current of the DC/DC converter is below the third limiting value, increases a power output of the DC/DC converter.
5. A method of controlling a DC power source having an output voltage with a first limiting value which should not be exceeded, and providing power from the DC power source to a DC/DC converter in which power conversion is controlled by a control strategy to convert output power of the DC power source to provide a desired output of the DC/DC converter, the method comprising:
- determining, by the DC/DC converter, whether the first limiting value of the output voltage of the DC power source has been exceeded, and if so, comparing, by the DC/DC converter, the value of an output current of the DC power source to a second limiting value; comparing, by the DC/DC converter, the value of an output current of the DC/DC converter to a third limiting value; and responsive to determining that the output current of the DC power source is below the second limiting value and the output current of the DC/DC converter is below the third limiting value, causing an increase in the power output of the DC/DC converter.
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Type: Grant
Filed: May 21, 2012
Date of Patent: Jun 13, 2017
Patent Publication Number: 20150229202
Assignee: Audi AG (Ingolstadt)
Inventors: Subbarao Varigonda (Manchester, CT), Daniel F. Orlowski (Manchester, CT)
Primary Examiner: Adolf Berhane
Assistant Examiner: Sisay G Tiku
Application Number: 14/401,821
International Classification: H02M 3/156 (20060101); H02M 3/157 (20060101); H02M 3/158 (20060101); H02M 1/32 (20070101); H01M 8/04828 (20160101); H01M 8/04537 (20160101); H02M 1/00 (20060101);